Conventionally, polymer membranes for water treatment are used for purifying water, for example, for removing turbidity from river water and groundwater, clarification of industrial water, treatment of wastewater and sewage, and as a pretreatment for seawater desalination, and the like.
Usually, such polymer membranes for water treatment are used as separation membranes in water treatment devices that utilize porous hollow fiber membranes formed from various polymer materials such as, for example, polystyrene (PS), poly(vinylidene fluoride) (PVDF), polyethylene (PE), cellulose acetate (CA), polyacrylonitrile (PAN), poly(vinyl alcohol) (PVA), polyimide (PI), and the like. In particular, polysulfone resins are frequently used due to their superior mechanical and chemical properties such as heat resistance, acid resistance, alkali resistance, and the like, and from the additional perspective of the ease of making a membrane.
By supplying contaminated water under pressure to micropores, porous hollow fiber membranes can remove contaminating substances from water by capturing only contaminating substances of a certain size or larger. In general, examples of the properties that are required in such polymer membranes for water treatment, in addition to the goal of separation characteristics, include having superior water permeability and superior physical strength, high stability toward a variety of chemical substances (namely, chemical resistance), less likelihood of adhesion of impurities during filtration (namely, superior antifouling properties), and the like.
For example, cellulose acetate fiber hollow-fiber separation membranes have been proposed that have comparatively high water permeability and are less likely to become contaminated even when used for long periods (see Patent Document 1).
However, this cellulose acetate hollow-fiber separation membrane has low mechanical strength and its chemical resistance is inadequate. Consequently, there is a problem in that when the separation membrane becomes contaminated, cleaning employs physical means or chemical means using chemical products and is extremely difficult.
Additionally, polymer membranes for water treatment have been proposed using hollow fiber membranes formed from poly(vinylidene fluoride) resin that have both superior physical strength and chemical resistance (see Patent Document 2). This polymer membrane for water treatment can be used by direct immersion in an aeration tank, and can be cleaned using various chemical agents even when contaminated.
However, poly(vinylidene fluoride) tends to have comparatively weak hydrophilic properties, with low antifouling properties.
The use of submersion-type MBRs (membrane bioreactors) have frequently been used in sewer water treatment in recent years (see Patent Documents 3 and 4). This submersion-type MBR is a method to obtain treated water by suction filtration using hollow fiber-type or flat-type water treatment membranes immersed in a biological water treatment tank, and the membrane surfaces are cleaned by continuous aeration to prevent the reduction in filtration efficiency when contaminants are deposited on the outer surface of the membrane.
However, the energy for the required aeration in immersion-type MBRs is associated with substantial electrical energy costs, which causes an increase in running costs.
In addition, depending on the particular application, water treatment is differentiated into inside-out filtration and outside-in filtration. For example, when the filtered liquid is already high-purity tap water, inside-out filtration is used and high-pressure water is supplied to hollow-fiber membranes with a small inner diameter. In this way, water treatment at a high filtration rate is possible. On the other hand, to prevent the occlusion inside the membrane by turbidity components when high-turbidity water is filtered, the operational method used for carrying out the water treatment employs low pressure using either a tubular membrane with a large pore diameter made from a composite material on a support frame or a flat membrane, and either an air flow or water flow is supplied to the membrane outer surface to prevent the deposition of the turbidity components on the membrane surface.
Furthermore, an inside-out type (outside the tank type) MBR has been proposed in which bio-treated water flows into a hollow-fiber membrane that is installed in a water bio-treatment tank and the filtration is carried out using internal pressure. In a water treatment membrane module that employs this inside-out mode MBR, a tubular-shaped water treatment membrane with an inner diameter of about 5 to 10 mm is used so that no occlusion will occur due to deposition of solids at the module end surface caused by bio-treated water that contains solids of varying sizes.
However, the treatment rate frequently cannot be accommodated due to problems with pressure resistance with flat-membrane filtration, and equipment other than the raw water pump and energy are required as a solution for preventing the deposition of turbidity components on the membrane surface.
Additionally, as a consequence of the increase in inner diameter of the water treatment membrane, treatment using a tubular membrane requires a firm support frame or an increase in membrane thickness to reduce the resistance to internal pressure during treatment. On the other hand, when using a support frame, generally back-pressure washing (backwashing) of the hollow-fiber membrane is carried out when the treatment rate decreases due to deposition of contaminants such as turbidity components and the like on the membrane surface, but damage can readily occur in this way when the tubular membrane that is attached to the support frame peels away. In particular, backwashing is practically impossible in an inside-out filtration-type tubular membrane that is suitable for the treatment of water with high suspended substances content. Consequently, in methods other than backwashing, a sponge ball is used to prevent a decrease in water permeation rate, a high internal flow rate is maintained, and the like, complication of the system and an increase in energy expenditures is currently unavoidable.
Moreover, when the water treatment membrane thickness is increased, this leads to the new problem of decreasing the footprint efficiency based on the water treatment rate.
Furthermore, in a method for manufacturing porous hollow-fiber membranes, conventionally, a resin solution comprising resin and solvent passes into a double-tube-type mold and a non-solvent is used to direct coagulation water to an interior part that will become a hollow part, and the non-solvent induced phase separation method (NIPS method) is applied to carry out phase separation by immersion of the outer part in a coagulation bath (for example, Patent Document 5). In this method, the resin solution leaving the mold is once brought into contact with air, and the solvent in the resin solution evaporates to form a skin layer. For this reason, the resin solution is submersed in the coagulation tank by dropping vertically due to gravity, and thereafter the membrane obtained by coagulation of the resin component in the coagulation tank is passed along a guide such as a roller, transferred to a different machine direction, and finally positioned horizontally in the machine direction and cut.
However, making the pore diameter in the porous hollow-fiber membrane larger can produce cracks, swelling, warping, uneven thicknesses, and the like. In addition, the take-up becomes difficult, and the manufacture of homogeneous hollow-fiber membranes, such as with a flattened membrane shape or the like, becomes extremely difficult, with the result that there are also problems in obtaining a polymer membrane for water treatment in which the abovementioned characteristics can be adequately achieved.